Anode for chromium plating



Patented July 9, .1929.

PATE T fOFFIGE.

I'BEDERICK M. BECKE'I I, OF NEW YORK, N. Y.

ANODE Eon cHRonnUM PLATING.

I No Drawing.

This invention relates to the art of electroplating with metallic chromium, and coinprises a soluble, chromi nn-containing cast anode for use in that operation,

Hcretofore such anodcshave commonly been made of cast chromiun of the highest purity which was commercially available, hereafter for brevity designated pure chromium.

Anodes of pure cast chromium present'a number of undesirable properties. Pure chromium is quite viscous even when heated to a temperature well above its melting point, so that anodes can be cast from it only in the simplest shapes. Such anodes are porous in varying degree, so that their rate of solution in the chromic acid electrolyte is irregular and impossible to predetermine. Furthermore, the crystal structure is such to affect most unfavorably the manner in which the anodes "di'ssolvez-l instead of dissolving smoothly and'regularly they tend to disintegrate, pieces of various sizes spalling or slonghing off and collecting at the bottom of ti e electrolytic cell, this involving not only 2n increase in anode consumption bug interfering with the operation of the cel These and perhaps other deficiencies of cast chromium anodes have been heretofore recognized, and have been summarized somewhat inaccurately by characterizing such anodes as brittle; It has been proposed to lessen this brittleness by additions of lead, cobalt, iron or nickel; and while some of these v additions in some proportions may diminish the so-called brittleness. of the anode, theyv have the disadvantage that eventually they either foul the bath or have an nndesirable'efiect upon the quality of the deosit. P I have found that the defects mentioned above can be overcome by incorporating in the metal a controlled proportion ofcarbon, the percentage being however materially below'that which characterizes'the known carbide cr G or which is present in the product resulting from the direct reduction .of

chromic oxid by carbon inthe'electric -fur-' nace. Proportions of carbon as low jas,0 .5% 'by weight exert a favorable effect, and proportions as high at 7.0% may be'us'ed. Theadvantages of the invention are attained in highest measure when the carbon content is- Application filed October 14, \1926. ,SerialJNo. 141,677

between 1 and 4 percent. As more fully explalned hereinafter 1t 1s likewise of advansition, in proportion depending on the particular results desired. In small pl'opol'i ion, say 1 to 2 percent incase of metal containing l-4 percent of carbon, silicon increases the fluidity of the molten metal' and in proves the crystalline structure of the cast article.

Anodes composed as above are'much superior to anodes of pure cast chromium. They are less brittle, and there is much less breakage by handling both before and after they are placed in the bath They have a better crystal structure or grain, which overcomes in large degree the difliculties of sp allingand sloughing. The composition in molten state is much more fluid than pure chromium, and its melting point lower;

so that anodes can be successively cast of' .tage to introduce silicon into the co1upof at a decidedly lower cost per unit of metallic chromium, as compared with anodes of pure cast chromium.

The electrolyte commonly and preferably usedconsists preponderantly of chromic acid in aqueous solution. Since chromic acid oxidizes carbon quite readily, undergoing reduction to the trivalent state, and since trivalent chromium in large proportions is known to interfere with the proper course of the electrolysis, it might naturally be as-. sinned that carbon in any substantial proportion would be, an "objectional constituent of' a chromium anode. I have found that this is in fact the case when the proportion of carbon 18 substantially greater than sevencast article. These efii'ects'are already noticeable with silicon additions as low as 0.25

percent; but in case these are the chief objectives I prefer to introduce one to two percent 'of silicon, as already stated.

2. It is characteristic of metallic chromiuin anodes, used in a chromic acid electrolyte, that they tend to d1ssolve more rapidly than corresponds to the deposition of metal at the cathode, with the result that the concentration of the bath tends to in-' crease progressively, andcorresponding irregularities are introduced into the operation. 'l.his difiici ilty has been overcome in the past by the simultaneous employment of soluble (chromium) and insoluble (lead) anodes, as disclosed in U. S. Patent 1,544,451

to I-lamlmechen. I have discovered that silicon, alloyed with the cromium in suflicient proportions, say above 5 or percent, re-

duces the rate of solution of the cromium in the electrolyte, such as aqueous solutions of 'increasing with increasing silicon content chromic acid, The rate of solution of the chromium dimin shes progressively although not proportionately as the percentage of sili-' con is increased, until at about 40 percent silicon and upward, the balance almost all chromium, the alloy functions practically like an insoluble-anode. -Accordingly it is possible, by properly adjusting the silicon content so to control the rate of solution of chromium as to ob t'ain the desired balance between the amount of metal dissolving at the anode and the amount deposited at the cathode, whereby the electrolyte remains within the limits of concentration suitable for efficient plating. It is of course impossible to specify a. definite silicon content which will accomplish this result in every case, because the cathode efficiency varies considerably with the type of electrolyte used, its concentration and temperature, the

current density, and other factors; but for use in baths consisting predominantly of chromic acid in aqueous solutlon, I find that the optimum percentages of silicon he between 10 and 35 percent; and in most cases between "and percent.

Silicon in the higher ranges, say upward of about 10 percent, acts to exclude carbon from the resulting alloy, its effect be brought to any desired point within the permissible limits of 0.57 percent. At the same time the rate of anodic solution of chromium is lessened, and the rate at which the concentration of the electrolyte increases is correspondingly lessened.

- Having regard to all three'of the abovedcscribed functions of silicon, as well as to the functions performed by carbon either in presence or absence of silicon, it will be understood that my present invention comprises broadly an anode of cast chromium metal containing about 0.5 to 7.0 percent of carbon, with or without silicon, the preferred carbon range being about 1 to 4 percent. The invention comprises also a cast chromium anode containing carbon as above, and in addition thereto silicon in proportion between about 0.25 percent and 35.0 percent according to the particular objectives to be attained. When the silicon is at or near the maximum mentioned the carbon will of course be at or near the minimum, by reason of the effect of silicon in excluding carbon from these alloys.

I do not claim herein a substantially carbon-free chromium anode containing from about 0.25 to percent of silicon, that subjcct matter being claimed in a copending application, Ser. No. 141,678, filed Oct. 14, 1926.

Iclaim:

1. A cast anode comprising chromium and about 0.5 to 7 percent of carbon.

2. A cast anode comprising chromium, silicon, and about 0.5 to 7 percent of carbon.

3. Acast anode comprising about 0.25 to 35.0 percent of silicon, about 0.5 to 7 percent of carbon, the balance substantially all chromium.

4. A cast anode comprising chromium and about 1 to 4 percent of carbon.

5. A cast anode comprising chromium, silicon, and about 1 to 4 percent of carbon.

6. In combination, an electrolyte consisting predominantly of an aqueous solution of chromic acid, and an anode comprising chromium, and about 0.5 to 7 percent of carbon.

7. In combination, an electrolyte consisting predominantly of an aqueous solution of chromicacid, and an anode comprising chromium, silicon, and about 0.5 to 7 percent of carbon.

8. In combination. an electrolyte consisting Iprmlominantly of an aqueous solution of chromic acid, and an anode comprising chromium. and carbon, substantially as described.

9. In combination. an electrolyte consisting predominantly of an aqueous solution of chromic acid, and an anode comprising chromium, carbon and silicon, substantially as described.

In testimony whereof, I aflix my signature.

FREDERICK M. BECKET. 

